Various kinds of thermoelectric modules, each having thermoelectric transducers, are known. The thermoelectric module cools and/or heats various kinds of apparatus by means of the thermoelectric transducers (so called peltier elements). For example, an optical transmission apparatus having the thermoelectric module is disclosed in JP 2004-253779A. The optical transmission apparatus disclosed in JP 2004-253779A cools a laser diode of an optical transmission module by thermoelectric transducers of the thermoelectric module. FIG. 18 is an explanation drawing schematically showing the known thermoelectric module and the optical transmission apparatus. Hereinafter, referring to FIG. 18, the known thermoelectric module and the optical transmission apparatus will be described.
A known optical transmission apparatus 100 shown in FIG. 18 includes a thermoelectric module 102, an optical transmission device 104, and a retaining member 103. The retaining member 103, formed in a box shape, retains the thermoelectric module 102 and the optical transmission device 104 therein. More specifically, the thermoelectric module 102 includes a first insulated substrate 120, a second insulated substrate 121, electrodes (not shown), and thermoelectric transducers 123.
The first insulated substrate 120 is firmly fixed at a bottom wall 124 of the retaining member 103 by means of adhering or soldering. Multiple electrodes (not shown) are formed on an upper surface of the first insulating substrate 120. The second insulated substrate 121 faces the first insulated substrate 120. Multiple electrodes (not shown) are formed on a lower surface of the second insulated substrate 121. Multiple thermoelectric transducers 123 are interposed between the second insulated substrate 121 and the first insulated substrate 120. Each thermoelectric transducer 123 is electrically connected to the corresponding electrode formed on the first insulated substrate 120, and also is connected with the corresponding electrode formed on the second insulated substrate 121. Further, a photodiode 125, comprising the optical transmission device 104, is disposed at the upper surface of the first insulated substrate 120. A laser diode 126, comprising the optical transmission device 104, is disposed at the upper surface of the second insulated substrate 121. When each thermoelectric transducer 123 is energized, a temperature of one surface of the thermoelectric transducer 123 rises and a temperature of another surface of the thermoelectric transducer 123 falls. The direction of the energization determines which side of the surface rises or falls the temperature. In the thermoelectric transducers 123 shown in FIG. 18, the temperature falls on a surface at the side of the second insulated substrate 121 and rises on a surface at the side of the first insulated substrate 120. Hence, the thermoelectric transducers 123 shown in FIG. 18 cool the laser diode 126 on the second insulated surface 121. The surface of each thermoelectric transducer 123, which rises the temperature, is fixed to the first insulated substrate 120. Thus, heat generated at the thermoelectric transducer 123 is dissipated to an exterior of the retaining member 103 through the first insulated substrate 120 and the bottom wall 124.
Generally, the thermoelectric module 102 is disposed at a limited space. For example, in the known optical transmission apparatus 100 shown in FIG. 18, the first and second insulated substrates 120 and 121 are respectively arranged in a manner that a surface, having larger area, is oriented to the bottom wall 124 of the retaining member 103. Hence, the number of the thermoelectric transducers 123, which are disposed on the first insulated substrate 120, is determined depending on an overall diameter of the bottom wall 124. Generally, the overall diameter of the bottom wall 124 in the optical transmission apparatus 100 is approximately 5 mm and is relatively small. Hence, a region of the first insulated substrate 120, on which the thermoelectric transducers 123 are disposed, is small and is not able to accommodate many thermoelectric transducers 123. Further, in the known optical transmission apparatus 100 shown in FIG. 18, the photodiode 125 is disposed on the first insulated substrate 120 as well as the thermoelectric transducers 123. Thus, the region of the first insulated substrate 120, on which the thermoelectric transducers 123 are disposed, is further limited. When a small number of the thermoelectric transducers 123 are disposed in the thermoelectric module 102, the heat exchange ability thereof is small. Accordingly, when a heat value of the laser diode 126 is large, the thermoelectric module 102 may be unable to cool the laser diode 126 by means of the thermoelectric module 102 effectively.
A need exists for a thermoelectric module which is not susceptible to the drawback mentioned above.